The radio-frequency (RF) electromagnetic spectrum, extending from below 1 MHz to above 100 GHz, represents a finite resource that is shared by variety of devices including devices operating using wireless communications standards, radar devices, television broadcasts, radio navigation and other RF devices. The increasing demand by consumers for higher data rates induces competition among RF devices for accessing the finite RF spectrum. Accordingly, appropriate federal agencies have recently recommended that 1000 MHz of federally-controlled RF spectrum should be freed or shared with the private industry in order to meet the ever growing need for wireless communications-based services.
A signal mixture represents a super-position of a plurality of individual signals with the addition of possible noise. Examples of signal mixtures occur in many applications. For example, when a plurality of RF devices such as radars and wireless communications devices are simultaneously operating over the same frequency spectrum, the received baseband signal at each individual device is a signal mixture that is a superposition of the signals from each RF device. If such signal is used as an input or source into receivers at radars or wireless communication devices, the performance of the radars and wireless communications will degrade relative to their actual performance potential when no super-position by external signals is present.
A special case of separation of mixtures which is more challenging are convolutive mixtures which may arise in several signal processing fields such as speech processing, music processing, sonar processing, radio communications processing, antenna array data processing, astronomical data processing, satellite imagery processing, functional brain image processing, etc. For example, in acoustic processing, such mixtures arise due to time delays resulting from sound propagation over space and the multipath induced from reflections of sound by different objects.
Radars are used for a variety of applications including air-traffic-control, weather forecasting, automotive collision avoidance systems, ground penetrating radars for finding underground resources, altimeters for elevation measurements, geophysical monitoring of resources by synthetic aperture radar (SAR) systems, etc. Studies have shown that the effect of wireless communications interference on radar systems may severely inhibit the performance of radar devices/systems. Therefore, conventionally, when a primary device (e.g., a radar device) operates in a given spectrum (e.g., frequency band), secondary devices such as devices communicating using wireless communications technologies, have not been allowed to operate in the given spectrum.
Various solutions have been proposed for enabling the use of “white spectrum” (e.g., RF spectrum used by primary devices) by the secondary devices. This means allowing secondary wireless devices to operate when the primary wireless device(s) are not active within a frequency band and geographical area. One such proposed solution is referred to as Dynamic Spectrum Access (DSA), with Dynamic Frequency Selection (DFS) being a particular example of the DSA solution.
Another proposed solution (not currently implemented or not implemented for spectrum sharing purposes) might be radar systems such as passive systems and multiple-input multiple-output (MIMO) radars to alleviate the spectrum congestion problem and make more spectrum available for use by wireless communications systems. However these systems are much more complex than the existing deployed radar systems. Furthermore, replacements of existing radar systems may be cost prohibitive and consequently such proposed systems are not currently feasible.
Therefore, more robust methods allowing for separation of simultaneously/overlappingly transmitted signals as well as simultaneous operation of wireless communications and radar devices/systems are desirable.